1. Field of the Invention
The invention relates to a catadioptric projection objective for projecting a pattern arranged in an object plane of the projection objective into an image plane of the projection objective with the formation of at least one real intermediate image at an image-side numerical aperture NA>0.7.
2. Description of the Related Prior Art
Projection objectives of this type are used in microlithography projection exposure systems for producing semiconductor components and other finely structured components. They are used to project patterns of photo masks or engraved plates, which in the following text will generally be designated masks or reticles, onto an object coated with a light-sensitive layer, with extremely high resolution on a reducing scale.
In this case, in order to generate finer and finer structures, it is necessary, firstly, to enlarge the image-side numerical aperture NA of the projection objective and, secondly, to use shorter and shorter wavelengths, preferably ultraviolet light with wavelengths of less than about 260 nm, for example 248 nm, 193 nm or 157 nm.
For wavelengths down to 193 nm, it is possible to operate with purely refractive (dioptric) projection systems, whose production can be managed easily because of their rotational symmetry about the optical axis. In order to achieve extremely small resolutions, however, it is necessary here to operate with extremely large numerical apertures NA of more than 0.8 or 0.9. In the case of dry systems with an adequately large, finite working distance (distance between the exit face of the objective and the image plane), these can be implemented only with difficulty. Refractive immersion systems have also already been proposed which, by using an immersion liquid of high refractive index between objective exit and image plane, permit values of NA>1.
For the aforementioned short wavelengths, however, it becomes more and more difficult to provide purely refractive systems with adequate correction of color errors (chromatic aberration), since the Abbe constants of suitable transparent materials are relatively close to one another. Therefore, for extremely high resolution projection objectives, use is predominantly made of catadioptric systems, in which refractive and reflective components, in particular therefore lenses and refractive mirrors, are combined.
When using projecting reflecting surfaces, it is necessary to use beam deflection devices if obscuration-free and vignetting-free projection is to be obtained. Both systems with geometric beam splitting by means of one or more wholly reflecting folding mirrors (deflection mirrors) and systems with physical beam splitting are known. Furthermore, planar mirrors can be used for folding the beam path. These are generally used in order to meet specific installation space requirements or in order to align the object and image plane parallel to each other. These folding mirrors are optically not absolutely necessary.
The use of a physical beam splitter, for example in the form of a beam splitter cube (BSC), with polarization-selective beam splitter surface makes it possible to implement projection objectives with an object field centered on the optical axis (one-axis systems). The disadvantage with such systems is that suitable transparent materials for the production of a beam splitter cube are available only to a limited extent in the required large volumes. In addition, the production of the polarization-selectively effective beam splitter layers presents considerable difficulties. An incomplete polarization-selective action can lead to the production of leakage transmission dependent on the angle of incidence and therefore to intensity inhomogeneities in the projection.
The disadvantage of systems with polarization-selective beam splitters can largely be avoided in systems with geometric beam splitting, that is to say when wholly reflective folding mirrors are used in the beam deflection device. Such a folding mirror permits the optical path leading to a concave mirror to be separated physically from the optical path leading away from the concave mirror. Many problems which can result from the use of polarised light are eliminated.
In the case of projection objectives with geometric beam splitting, various folding geometries are possible, there being specific advantages and disadvantages, depending on the course of the light path between object field and image field.
U.S. Pat. No. 6,195,213 B1 shows various embodiments of projection objectives with geometric beam splitting for projecting a pattern of a mask arranged in an object plane of the projection objective into an image plane of the projection objective with the formation of a single, real intermediate image. The projection objectives, which reach image-side numerical apertures up to NA=0.75, have a catadioptric first objective part, which is arranged between the object plane and the image plane and has a concave mirror and a beam deflection device, and also a dioptric second objective part, which is arranged between the first objective part and the object plane. The elements of the first objective part used for forming the intermediate image are designed in such a way that the intermediate image lies optically and geometrically in the vicinity of the first folding mirror. The beam deflection device has a first folding mirror, which is arranged in the beam path between the concave mirror and the image plane. In these systems, the first folding mirror is arranged in such a way that light coming from the object plane falls firstly on the concave mirror of the first objective part before it is reflected by the latter to the first folding mirror. From the latter, it is deflected by 90° and reflected to a second folding mirror, which deflects the radiation coming from the first folding mirror once more through 90° in the direction of the image plane. This beam guidance leads to an h-shaped structure of the system, for which reason this folding geometry is also designated h-folding. The projection objective has only one concave mirror.
Accommodated in the space between object plane and first folding mirror are a plurality of lenses used for the optical correction. The region between the folding mirrors is free of lenses, which is intended to permit a compact design. Therefore all the lenses and the concave mirror are arranged in objective parts which can be aligned vertically, which is intended to achieve a structure which is stable with respect to the influences of gravity.
In U.S. Pat. No. 5,969,882 (corresponding to EP-A-0 869 383), other projection objectives with h-folding and only one concave mirror are described, in which lenses are arranged in the space between object plane and first folding mirror. In embodiments in which the first and the second folding mirror are configured as reflective surfaces of a deflection prism, the region between the folding mirrors is free of any refractive power.
European patent EP 0 604 093 B1 and U.S. Pat. No. 5,668,673 connected thereto via a common priority show catadioptric projection objectives with relatively low numerical apertures of NA<0.5, in which the object field is projected into the image field with the aid of two concave mirrors, forming a single real intermediate image. Embodiments with different, partly complex folding geometries are shown, in some embodiments a first beam section running from the object plane to a concave mirror and a second beam section running from this concave mirror to the image plane crossing. The complex folding geometries with a large number of optical components physically close to one another mean that considerable mechanical and mounting problems may be expected in the practical implementation of such designs. A transfer of the design concepts into the area of higher numerical apertures appears not to be practical, on account of the associated greater maximum beam diameters and the correspondingly increasing maximum lens diameters.
EP 0 989 434 (corresponding to U.S. Pat. No. 6,496,306) shows projection objectives with a beam deflection device formed as a mirror prism. The mirror prism forms the first folding mirror for the deflection of the radiation coming from the object plane to the concave mirror, and a second folding mirror for the deflection of the radiation reflected from the concave mirror to the second objective part, which contains only refractive elements. The catadioptric first objective part forms a real intermediate image, which is located freely accessibly at a distance behind the second reflecting surface. The single concave mirror is fitted in a side arm of the projection objective which projects transversely with respect to the vertical direction when installed and which is also designated a “horizontal arm” (HOA). On account of the 1-form geometry of the beam path, such a folding geometry is also designated “1-folding”. Other projection objectives with only one concave mirror and 1-folding are described, for example, in DE 101 27 227 (corresponding to U.S. patent Application US 2003/021040) or the international patent Application WO 03/050587.
Systems with geometric beam splitters have the disadvantage, caused by the principle, that the object field is arranged eccentrically with respect to the optical axis (extra-axial system or off-axis system). This places high requirements on the correction of image errors since, in such a projection system, as compared with on-axis systems, a larger usable field diameter has to be corrected adequately with the same object field size. This larger field area, including the object field, will also be designated the “superfield” in the following text.
Optimization of the size of the superfield becomes more and more difficult as the numerical aperture of the projection objective increases, since the clearances for the arrangement and dimensioning of optical components and here, in particular, the folding mirror, become smaller and smaller, with the limiting condition of vignetting-free projection. In addition, the mechanical mounting of the optical components increasingly presents difficulties the more complex their relative arrangement to one another is.